[BUG] Fable 5.1 requires unstable release of Claude Code

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

All attempts to use Fable 5.1 (`claude-fable-5-1`) in the current [stable release of Claude Code](https://code.claude.com/docs/en/setup#configure-release-channel) fail deterministically with the following error message:

```text
⏺ API Error: 400 Claude Code 2.1.236 does not support this model; version 2.1.251 or
newer is required. Run 'claude update', or update the Claude desktop app, then
try again.
```

### What Should Happen?

Fable 5.1 requests should succeed in the current stable release of Claude Code. As of this moment, that is `2.1.236`:

```text
❯ date; npm view @anthropic-ai/claude-code dist-tags.stable
Tue Sep 1 18:07:30 CDT 2026
2.1.236
```

### Error Messages/Logs

```shell
`
2026-09-01T22:55:08.166Z [DEBUG] Acquired PID lock for 2.1.236 (PID 24426)
...
2026-09-01T22:55:10.796Z [DEBUG] [engine] send set_max_thinking_tokens max=null
2026-09-01T22:55:10.796Z [DEBUG] [engine] send set_model model=claude-fable-5-1
...
2026-09-01T22:55:17.794Z [DEBUG] [API:timing] dispatching to firstParty model=claude-haiku-4-5-20251001
2026-09-01T22:55:17.795Z [DEBUG] [API REQUEST] /v1/messages x-client-request-id= source=generate_session_title
...
2026-09-01T22:55:17.797Z [DEBUG] [engine] turn 1 start
...
2026-09-01T22:55:17.803Z [DEBUG] [API:timing] dispatching to firstParty model=claude-fable-5-1
2026-09-01T22:55:17.803Z [DEBUG] [API REQUEST] /v1/messages x-client-request-id= source=repl_main_thread
...
2026-09-01T22:55:18.240Z [ERROR] API error (attempt 1/11): 400 400 {"type":"error","error":{"type":"invalid_request_error","message":"Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.","details":{"error_code":"claude_code_version_too_old"}},"request_id":""}
2026-09-01T22:55:18.240Z [WARN] [server-fallback] 400 attributed (unattributed) — stripping and retrying
...
2026-09-01T22:55:18.241Z [DEBUG] [API:timing] dispatching to firstParty model=claude-fable-5-1
2026-09-01T22:55:18.241Z [DEBUG] [API REQUEST] /v1/messages x-client-request-id= source=repl_main_thread
2026-09-01T22:55:18.305Z [DEBUG] Stream started - received first chunk
2026-09-01T22:55:18.305Z [DEBUG] [API:timing] first byte after 511ms
...
2026-09-01T22:55:18.537Z [ERROR] API error (attempt 1/11): 400 400 {"type":"error","error":{"type":"invalid_request_error","message":"Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.","details":{"error_code":"claude_code_version_too_old"}},"request_id":""}
2026-09-01T22:55:18.537Z [WARN] [server-fallback] 400 attributed (unattributed) — stripping and retrying
...
2026-09-01T22:55:18.538Z [DEBUG] [API REQUEST] /v1/messages x-client-request-id= source=repl_main_thread
2026-09-01T22:55:18.795Z [ERROR] API error (attempt 1/11): 400 400 {"type":"error","error":{"type":"invalid_request_error","message":"Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.","details":{"error_code":"claude_code_version_too_old"}},"request_id":""}
2026-09-01T22:55:18.802Z [ERROR] Error in API request: 400 {"type":"error","error":{"type":"invalid_request_error","message":"Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.","details":{"error_code":"claude_code_version_too_old"}},"request_id":""}
2026-09-01T22:55:18.802Z [ERROR] API error x-client-request-id= (give this to the API team for server-log lookup)
2026-09-01T22:55:18.804Z [ERROR] Error: Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.","details":{"error_code":"claude_code_version_too_old"}},"request_id":""}
at generate (/$bunfs/root/cli:39:39337)
at makeRequest (/$bunfs/root/cli:90:8063)
at processTicksAndRejections (native:7:39)
2026-09-01T22:55:18.805Z [DEBUG] [engine] turn 1 end (turns=1 usage in=896 out=11 cost=$0.0010 api=648ms stop=stop_sequence resultLen=0)
2026-09-01T22:55:18.805Z [ERROR] [engine] turn ended in error: API Error: 400 Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.
...
2026-09-01T23:05:22.843Z [DEBUG] Protecting locked version from cleanup: 2.1.227
`
```

### Steps to Reproduce

1. `claude install stable`
2. `claude --model claude-fable-5-1 --print 'what is your knowledge cutoff?'`
3. observe the following error message and a non-zero exit code (`1`): `API Error: 400 Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.`

### Claude Model

Other

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

2.1.236

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

iTerm2

### Additional Information

_No response_

Start by reproducing the failure with `claude install stable` and `claude --model claude-fable-5-1 --print 'what is your knowledge cutoff?'`, then inspect the CLI entry point associated with the bundled `/$bunfs/root/cli` stack trace. The issue names no source file or test; done means the stable release accepts Fable 5.1 without the reported 400 error and the command exits successfully.

Written by the indexing model from the issue text.